The US Justice Department said on Tuesday it has sued to stop United Airlines from acquiring 24 takeoff and landing slots at Newark Liberty International Airport from Delta Airlines.

The US Justice Department said on Tuesday it has sued to stop United Airlines from acquiring 24 takeoff and landing slots at Newark Liberty International Airport from Delta Airlines.

“A slot is essentially a license to compete at Newark,” said Assistant Attorney General Bill Baer in a statement. “United already holds most of them, and as a result, competition at Newark is in critically short supply.”

The lawsuit, filed in U.S. District Court for New Jersey in Newark, alleges that the acquisition would lead to higher fares and fewer choices for travelers.

The Justice Department said that United already controls 73 percent of the slots, which are authorizations from the Federal Aviation Administration to take off from or land at the airport. It also said United allegedly “grounds” as many as 82 slots each day at Newark, limiting flight opportunities.

The federal government has been concerned about a possible monopoly since 2010, when United, the third largest airline in the world by revenue, divested 36 of its slots to Southwest Airlines to help win approval of its merger with Continental Airlines.

The current transaction is United’s third attempt to “reverse the benefits” of that divestiture, the department said.

Both United and Delta did not immediately respond to a request for comment.

United’s stock, which had traded as high as $60.85 earlier in the session, dropped to as low as $59.03 before clawing part way back.